A Shack-Hartmann sensor that is a device for measuring distortions or aberrations of a wavefront of light reflected by a specific region in an astronomical telescope, an optometer, or the like is generally used to measure a shape of a surface in the specific region by using the measured distortions or aberrations of the wavefront.
However, a Shack-Hartmann sensor has a limitation in that a change in an overall thickness or an overall height of an object may not be measured. For example, when a thickness difference between wafers having different thicknesses and stacked on a reference surface such as a surface of a stage is to be measured, a Shack-Hartmann sensor cannot measure the thickness difference or may have great difficulty in measuring the thickness difference. This is because a size of probe light emitted to the object has to be large enough to include both the wafers and the reference surface and a height difference between the reference surface and a measurement surface when the reference surface becomes a reflective surface for the probe light should not exceed a measurement limit (e.g., about 30 times a wavelength of the probe light) of the Shack-Hartmann sensor.